Sensors are employed in a wide range of technological applications including automotive, industrial and consumer applications. These applications need sensors that operate reliably according to specified performance characteristics. For this reason, sensors are calibrated to offset deviations in sensor performance and to ensure that measurements are performed accurately. The sensor industry has developed self-calibrating sensors to reduce calibration time and cost. Self-calibrating sensors include some Hall effect sensors, magneto-resistive sensors and other suitable sensors.
Usually, in Hall effect sensors, a constant current is provided to a Hall element or plate and a magnetic field is applied perpendicular to the current flowing through the Hall plate. Charge carriers in the Hall plate are deflected due to the Lorentz force to create a Hall voltage that is perpendicular to both the magnetic field and the current flow. This Hall voltage can be measured and is directly proportional to the magnetic field. Hall effect sensors are used for speed, rotational speed, linear position, linear angle and position measurements in automotive, industrial and consumer applications.
Magneto-resistive (XMR) sensors, typically, include a supporting magnet and one or more XMR sensor elements for measuring a magnetic field. The supporting magnet and the XMR sensor elements are in a fixed position relative to each other. The XMR sensor elements do not usually operate in their saturation range and the supporting magnet provides a back bias magnetic field that is superimposed on the XMR sensor elements to stabilize the transfer characteristic of the XMR sensor elements. As the position of a detected object changes relative to the source of the magnetic field, the magnetic field produces a proportional voltage signal in the XMR sensor elements. Suitable XMR sensor elements include anisotropic magneto-resistive (AMR) sensor elements, giant magneto-resistive (GMR) sensor elements, tunneling magneto-resistive (TMR) sensor elements, and colossal magneto-resistive (CMR) sensor elements. XMR sensors can be used as proximity sensors, motion sensors, position sensors, or speed sensors.
Typically, self-calibrating sensors with switching outputs are self-adjusted to a switching point. Minimum and maximum values of the incoming signal are obtained and the switching point is calculated from them. For example, the optimum switching point may be exactly half-way between the maximum and minimum values. Usually, a fast regulation or tracking loop follows the incoming signal to determine an offset signal, which is summed with the incoming signal to adjust the incoming signal to the switching point. In the adjusted condition, the offset signal is a DC signal that is overlaid on the incoming signal. A main comparator receives the incoming signal or a signal based on the incoming signal and provides an output that switches from one state to another.
The resolution of the tracking loop must be high enough to track the incoming signal and provide the offset signal. Also, sometimes the tracking loop circuitry cannot follow offset jumps in the incoming signal, where the offset jumps may be caused by external disturbances or the changing of a sensor air-gap. In addition, the tracking loop often includes circuitry that introduces errors into the incoming signal, such as amplifiers that introduce offset error, gain error and noise, and comparators that introduce offset error.
For these and other reasons there is a need for the present invention.